1. Field of the Invention
The present invention relates to a method and apparatus for converting the resolution of an image signal.
2. Description of the Related Art
The resolution of an image signal refers to the number of picture elements or pixels (dots) in the image, measured in the horizontal and vertical directions. Resolution conversion is necessary when an image signal input in a format having a prescribed resolution is displayed on a display device having a different resolution. Such conversion is frequently needed when an image captured by a camera is displayed on a dot matrix display device such as a liquid crystal display (LCD), a plasma display panel (PDP), or a digital micro-mirror device (DMD). Similar conversion is needed when an image is displayed at a reduced size in a window inside another image (picture in picture), or when multiple images are displayed at reduced sizes on the same screen (picture and picture). Resolution conversion is also referred to as scaling.
Conventional scaling apparatus has, for example, an image signal memory, an interpolation coefficient memory, a scaling control unit, a memory address generator, and an interpolation unit. The apparatus operates by reading a source image signal from the image signal memory, performing interpolation, and outputting the interpolated pixels as a scaled image signal. Interpolation may either increase or decrease the number of pixels in the image; that is, the image may be either enlarged or reduced. The vertical and horizontal scaling factors may accordingly be either greater than or less than unity.
A scaling factor can be represented as a fraction P/Q, where Q corresponds to the height or width of the source image and P corresponds to the height or width of the scaled image, P and Q being positive integers. The scaling process can be carried out in a simple way by enlarging the image by a factor of P and reducing the enlarged image by a factor of Q. A disadvantage of this simple scheme is that the enlargement process must be carried out at P times the sampling frequency of the source image, which may not be feasible for large values of P.
A solution to this problem, disclosed in Japanese Unexamined Patent Application Publication No. 9-181970 (1997), is to prestore P sets of interpolation coefficients in a register memory, and select the appropriate interpolation coefficients for each pixel according to phase information indicating the position of the interpolated pixel relative to the adjacent source pixels used as reference pixels for interpolation. The image can then be scaled by a factor of P/Q in a single operation, without having any circuits operate faster than the sampling frequency of the source image or converted image. A disadvantage of this scheme is that when P is large, a large register memory is needed to store the interpolation coefficients.
For many values of P, the necessary amount of register memory can be reduced by the use of an iterated scheme disclosed in Japanese Unexamined Patent Application Publication No. 2001-229372, in which P/Q is expressed as a product of two fractions having smaller numerators and denominators and scaling is carried out twice. To scale an image by a factor of 128/225, for example, the image may be scaled first by a factor of 8/9, then by a factor of 16/25, so that no more than sixteen sets of interpolation coefficients have to be stored at a time. This scheme also simplifies the generation of the addresses of reference pixels in the source image, enabling the scaling operation to be carried out by a general-purpose processor such as a digital signal processor with relative simple built-in memory control circuitry, instead of by a specialized scaling processor.
One disadvantage of the iterated scaling scheme is the need for a large frame memory to store the intermediate image created by the first scaling operation, and the need to transfer large amounts of pixel data to and from this frame memory, with attendant delays.
Another disadvantage is that iterated scaling is applicable only when P can be factored. For example, iterated scaling cannot be used to enlarge an image by a factor of 463/175, because 463 is a prime number.
It would be desirable to have a method and apparatus that can scale an image by an arbitrary scaling factor without the need to operate at a high sampling frequency, store a large number of interpolation coefficients, or transfer a large amounts of intermediate pixel data.